Mathematics (chaos theory) shows that meteorologists will
never be able to predict the weather for the UK really accurately beyond about 3 days, with a reasonably decent chance of getting it right for a further day or two beyond that. Thereafter, up until about 10 days ahead, they can suggest what the best or worst might be but add little more detail. (For example, the BBC weather staff are currently predicting that the maximum temperature in London next Friday will be between 14C and 23C, and that there might be some showers).
For areas with more stable weather conditions (e.g. Athens or Melbourne) forecasts can go a it further ahead but that's largely because forecasters know (from historic records) that the whole of a month is likely to dry and hot anyway.
The Met Office currently attempts to give a rough guide to conditions over the next 30 days
http://www.metoffice.gov.uk/weather/uk/uk_forecast_alltext.html
but it looks like that might disappear soon.